Furnace with primary and secondary heat exchangers

ABSTRACT

A furnace that includes a heater configured to heat a fluid and discharge a heated fluid flow, a first heat exchanger at least partially positioned in a air duct and in fluid communication with the heater to receive the heated fluid flow, and a second heat exchanger at least partially positioned in the air duct and in fluid communication with the first heat exchanger so that the heated fluid flow flows through the second heat exchanger after flowing through the first heat exchanger. The first heat exchanger and the second heat exchanger are arranged in the air duct so that the flow of air travels through the first heat exchanger to transfer heat between the heated fluid flow and the flow of air, and then through the second heat exchanger after the first heat exchanger to transfer heat between the heated fluid flow and the flow of air.

BACKGROUND

The present invention relates to a furnace that is utilized to heat a flow of air.

Furnaces can be used to heat a flow of air partially drawn from outside air and partially drawing from the space conditioned or heated by the furnace. The flow of air is passed through a heat exchanger to heat the flow of air. The heat exchanger can receive heated combustion air from a burner to heat the flow of air. After passing through the heat exchanger the heated flow of air is discharged to a conditioned space while the combustion air can be discharged to the atmosphere.

SUMMARY

In one embodiment, the invention provides a furnace configured to heat a flow of air and discharge the heated flow of air into a conditioned space. The furnace includes an air duct including an air inlet and an air outlet, and the air duct defines an air flow path from the air inlet to the air outlet. The air duct is configured to direct the flow of air along the air flow path from the air inlet to the air outlet. The furnace further includes a heater configured to heat a fluid and discharge a heated fluid flow, a first heat exchanger at least partially positioned in the air duct and in fluid communication with the heater to receive the heated fluid flow, and a second heat exchanger at least partially positioned in the air duct and in fluid communication with the first heat exchanger so that the heated fluid flow flows through the second heat exchanger after flowing through the first heat exchanger. The first heat exchanger and the second heat exchanger are arranged in the air duct so that the flow of air travels through the first heat exchanger to transfer heat between the heated fluid flow and the flow of air, and then through the second heat exchanger after the first heat exchanger to transfer heat between the heated fluid flow and the flow of air.

In another embodiment the invention provides a method of operating a furnace including heating a fluid with a heater, directing the heated fluid through a first heat exchanger, directing a flow of air through the first heat exchanger, transferring heat from the heated fluid to the flow of air in the first heat exchanger, directing the heated fluid through a second heat exchanger after directing the heated fluid through the first heat exchanger, directing the flow of air through the second heat exchanger after directing the flow of air through the first heat exchanger, transferring heat from the heated fluid to the flow of air in the second heat exchanger.

Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a furnace according to an embodiment of the invention with a portion of the furnace removed to show an interior of the furnace.

FIG. 2 is first perspective view of a portion of the furnace of FIG. 1.

FIG. 3 is a second perspective view of the portion of the furnace of FIG. 2.

FIG. 4 is a side view of the portion of the furnace of FIGS. 2 and 3.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

Also, it is to be understood that phraseology and terminology used herein with reference to device or element orientation (such as, for example, terms like “central,” “upper,” “lower,” “front,” “rear,” and the like) are only used to simplify description of the present invention, and do not alone indicate or imply that the device or element referred to must have a particular orientation. In addition, terms such as “first” and “second” are used herein for purposes of description and are not intended to indicate or imply relative importance or significance.

FIG. 1 illustrates a furnace 10. In one embodiment, the furnace 10 is mounted to a roof of a building or other structure to provide heated air to a conditioned space or room within the building. In other embodiments, the furnace 10 can be mounted to other suitable locations on the building, such as an exterior wall or the like. In yet other embodiment, the furnace 10 can be mounted to a frame and ducts can be used to fluidly couple the furnace 10 to the conditioned space or room within the building.

Referring to FIGS. 1 and 2, the illustrated furnace 10 includes an air duct 14, an air handling unit 16, a heater 18, a first heat exchanger 20, and a second heat exchanger 22. The air duct 14 includes an air inlet 26 and an air outlet 28. The illustrated air duct 14 is formed by sheet metal panels 30 that can be cut, bent, and then fastened together to form the air duct 14. It should be noted that sheet metal panels 30 have been removed from a rear side 32 of the furnace 10 in the illustration in FIG. 1 to show the heat exchanger 20 and inside the air duct 14 in the illustration of FIG. 1.

The air duct 14 forms an air flow path to direct a flow of air from the inlet 26 and through the first heat exchanger 20 and the second heat exchanger 22 and then through the outlet 28. In the illustrated embodiment, the air inlet 26 is positioned outside the building or structure so that all (i.e., 100 percent) of the air that flows through the air duct 14 is outside or ambient fresh air. In other embodiments, a majority (i.e., greater than 50 percent) of the air that flows through the air duct 14 is outside air and the remainder of the air that flows through the air duct can be air that is circulated from the conditioned space (i.e., inside air). The outlet 28 is positioned so that the air duct 14 discharges the flow of air into the conditioned space or building. Additional ducts or the like can be attached to the outlet 28 to facilitate transporting the flow of air into the conditioned space or building.

With continued reference to FIG. 1, the illustrated air handling unit 16 includes a centrifugal fan 34 and an electric motor 36. The motor 36 is coupled to the fan 34 using a drive system so that operation of the motor 36 rotates the fan 34 to draw the flow of air into the duct 14 through the inlet 26.

Referring to FIG. 2, the illustrated heater 18 includes a plurality of burners 40, a fuel supply header 42, a fuel supply line 44, a first valve 46, and a second valve 48. In the illustrated embodiment, each of the burners 40 is an inshot burner. In other embodiments, other types of burners can be used, such as ribbon burners, strip burners, power burners, etc. The burners 40 are in fluid communication with the fuel supply header 42 and the header 42 is in fluid communication with the fuel supply line 44. The first valve 46 is operable to open and close the fuel supply line 44 to allow fuel, such as natural gas or propane, to flow through the line 44 or stop the flow of fuel through the line 44 to the burner 40. The second valve 48 is downstream from the first valve 46 along the line 44, and the second valve 48 is a modulating valve that allows the amount of fuel flow through the line 44 and to the burners 40 to be adjusted to a desired amount.

Referring to FIGS. 2 and 3, the illustrated first heat exchanger 20 is a tubular heat exchanger including a plurality of tubes 52 each having a tube inlet 54 aligned with one of the burners 40, and the tube inlet 54 receives heated fluid from the adjacent burner 40 that is a by-product of the combustion of the fuel at the burner 40. The illustrated tubes 52 are serpentine and define a general “S” configuration. Each of the tubes 52 also has a tube outlet 56 that discharges the heated fluid (i.e., heated gas in the illustrated embodiment) from the first heat exchanger 20. In other embodiments, other suitable types of heat exchangers can be used, including, fin tube, plate, clam shell, and drum type heat exchangers.

The illustrated second heat exchanger 22 is a fin tube heat exchanger including tubes 58 and fins 60 located between the tubes 58. The tubes 58 each include an inlet end 64 (FIG. 2) and an outlet end 66 (FIG. 3). The inlet end 64 includes an aperture that receives the discharged heated gas from the tube outlets 56 of the first heat exchanger 20. In the illustrated embodiment, a turn-around header 68 is coupled to the tube outlets 56 and the inlet ends 64 to direct the discharged heated gas from the first heat exchanger 20 to the second heat exchanger 22. In other embodiments, the second heat exchanger can be other suitable types of heat exchangers, including, tubular, plate, clam shell and drum type heat exchangers.

Referring to FIG. 2, a discharge header 70 is coupled to the outlet ends 66 of the tubes 58 of the second heat exchanger 22 to receive and collect the heated gas from the second heat exchanger 22 after it flows through the second heat exchanger 22. A condensate outlet 72 is in fluid communication with the discharge header 70 to collect and discharge any condensate that may form in the heat exchangers 20 and 22 and the headers 68 and 70. Condensate traps 76 (FIG. 1) are fluidly coupled to the condensate outlet 72 to control the discharge of condensate from the furnace 10.

With reference to FIGS. 2 and 4, a combustion air handling unit 80 is in fluid communication with the discharge header 70. The air handling unit 80 includes a centrifugal fan 82 and an electric motor 84 that is operable to drive the fan 82. The fan 82 includes an inlet that is in fluid communication with the discharge header 70 and an outlet 88 that is in fluid communication with exhaust outlets 92 (FIG. 1). The motor 84 drives the fan 82 to draw combustion air through the inlets 54 of the tubes 52 and draw the heated gas through the tubes 54 and 58 of the heat exchangers 20 and 22 and discharge the heated gas through the exhaust outlet 92 of the furnace 10.

In operation, the air handling unit 16 draws a flow of air through the air inlet 26. The air duct 14 directs the flow of air through the first heat exchanger 20 and then through the second heat exchanger 22 generally in the direction of arrows 96 in FIG. 4. Meanwhile, the fuel supply line 44 supplies fuel to the burners 40 where the fuel is ignited and burned and the burners 40 discharge the heated gas into the tubes 52 of the first heat exchanger 20. The heated gas travels through the tubes 52 and 58 generally in the direction represented by the arrows 98 in FIG. 4. Heat is transferred from the heated gas to the flow of air in the first heat exchanger 20 as the flow of air travels over and around the tubes 52. The flow of air, which has been heated in the first heat exchanger, a primary heat exchanger in the illustrated embodiment, then flows through the second heat exchanger 22. The flow of air 22 travels between the tubes 58 and through the fins 60 of the second heat exchanger 22. Meanwhile, the heated combustion air flows from the first heat exchanger 20 into the turn-around header 68 and into the tubes 58 of the second heat exchanger 22. In the second heat exchanger 22, additional heat is transferred from the heated gas to the flow of air to further heat the flow of air. After flowing through the second heat exchanger 22, the flow of air is discharged through the outlet 28 of the duct 14 and supplied to the conditioned space or building, and the heated gas travels through the fan 82 and is discharged to the atmosphere via the exhaust outlets 92.

As the heated gas is cooled by the flow of air in the first heat exchanger 20 and the second heat exchanger 22, condensation may be formed. For example, in one embodiment, the heated gas is cooled by the flow of air in the second heat exchanger 22 from a first temperature above a dew point temperature of the heated gas to a second temperature below the dew point temperature of the gas, which causes condensation to form in the second heat exchanger 22. The condensate travels by gravity through the condensate outlet 72 and then through the traps 76 where it is discharged from the furnace 10. In the illustrated embodiment, the flow of air is first heated by the primary heat exchanger 20 which is directly downstream from the burners 40 and then the flow of air is heated by the secondary heat exchanger 22, which is downstream from the primary heat exchanger 20. Such an arrangement inhibits condensate from forming and then freezing in the primary heat exchanger 20 due to the relatively high temperatures adjacent the burners 40. When the flow of air drawn through the air inlet 26 is at or below 0 Celsius or the freezing temperature of water this arrangement may be particularly beneficial. Any condensate that forms in the secondary heat exchanger 22 is inhibited from freezing because the flow of air has already been heated to a temperature above the freezing temperature of water by the first heat exchanger 20. Such an arrangement is particularly useful in applications where a majority of the air drawn through the air inlet 26 is ambient or outdoor air, which may be at temperatures below the freezing temperature of water.

Various features and advantages of the invention are set forth in the following claims. 

1. A furnace configured to heat a flow of air and discharge the heated flow of air into a conditioned space, the furnace comprising: an air duct including an air inlet and an air outlet, the air duct defining an air flow path from the air inlet to the air outlet, the air duct configured to direct the flow of air along the air flow path from the air inlet to the air outlet; a heater configured to heat a fluid and discharge a heated fluid flow; a first heat exchanger at least partially positioned in the air duct and in fluid communication with the heater to receive the heated fluid flow; and a second heat exchanger at least partially positioned in the air duct and in fluid communication with the first heat exchanger so that the heated fluid flow flows through the second heat exchanger after flowing through the first heat exchanger, wherein the first heat exchanger and the second heat exchanger are arranged in the air duct so that the flow of air travels through the first heat exchanger to transfer heat between the heated fluid flow and the flow of air, and then through the second heat exchanger after the first heat exchanger to transfer heat between the heated fluid flow and the flow of air.
 2. The furnace of claim 1, wherein a majority of the flow of air entering the air inlet is outdoor air.
 3. The furnace of claim 1, wherein the flow of air is discharged from the air outlet into a building.
 4. The furnace of claim 1, wherein the first heat exchanger is positioned outdoors.
 5. The furnace of claim 1, wherein the second heat exchanger is positioned outdoors.
 6. The furnace of claim 1, further comprising at least one condensate reservoir fluidly coupled to the second heat exchanger and positioned to receive condensate from the heated fluid flow in the second heat exchanger.
 7. The furnace of claim 1, further comprising an air handling unit including a motor and a centrifugal fan, the motor is configured to rotate the centrifugal fan and the centrifugal fan is configured to direct the flow of air along the air flow path.
 8. The furnace of claim 1, wherein the heater includes at least one burner.
 9. The furnace of claim 8, wherein the at least one burner is an inshot burner, and wherein the first heat exchanger includes a plurality of tubes in which the heated fluid flows.
 10. The furnace of claim 9, wherein the second heat exchanger includes a plurality of tubes and a plurality of fins, and wherein the heated fluid flows through the plurality of tubes of the second heat exchanger and the flow of air flows through the plurality of fins.
 11. The furnace of claim 1, wherein the first and the second heat exchangers are positioned outdoors, and wherein the air inlet is positioned outdoors.
 12. A method of operating a furnace comprising: heating a fluid with a heater; directing the heated fluid through a first heat exchanger; directing a flow of air through the first heat exchanger; transferring heat from the heated fluid to the flow of air in the first heat exchanger; directing the heated fluid through a second heat exchanger after directing the heated fluid through the first heat exchanger; directing the flow of air through the second heat exchanger after directing the flow of air through the first heat exchanger; and transferring heat from the heated fluid to the flow of air in the second heat exchanger.
 13. The method of claim 12, further comprising inhibiting freezing of condensate from the flow of air in the second heat exchanger by transferring heat from the heated fluid to the flow of air in the first heat exchanger prior to directing the flow of air through the second heat exchanger.
 14. The method of claim 12, further comprising moving outdoor air into the first heat exchanger prior to directing the flow of air through the first heat exchanger.
 15. The method of claim 12, further comprising discharging the flow of air into a building after directing the flow of air through the second heat exchanger.
 16. The method of claim 12, wherein the flow of air through the second heat exchanger is warmer than the flow of air through the first heat exchanger.
 17. The method of claim 12, wherein the fluid in the first heat exchanger is warmer than the fluid in the second heat exchanger.
 18. The method of claim 12, further comprising positioning the first heat exchanger outdoors.
 19. The method of claim 12, further comprising positioning the second heat exchanger outdoors.
 20. The method of claim 12, further comprising fluidly coupling at least one condensate reservoir to the second heat exchanger to receive condensate from the heated fluid in the second heat exchanger.
 21. The method of claim 12, wherein heating the fluid with a heater includes igniting a gas in a burner.
 22. The method of claim 12, wherein transferring heat from the heated fluid to the flow of air in the first heat exchanger includes raising a temperature of the flow of air from a first temperature less than or equal to 0 degrees Celsius to a second temperature above 0 degrees Celsius.
 23. The method of claim 12, wherein the heated fluid has a dew point temperature in the second heat exchanger, wherein transferring heat from the heated fluid to the flow of air in the second heat exchanger includes lowering a temperature of the heated fluid from a first temperature above the dew point temperature to a second temperature below the dew point temperature. 